State of the art wireless communication systems, such as 2.5 and 3rd generation networks, based on standards such as United States Digital Cellular (USDC IS-54, IS-94 and ID-136), Global System for Mobile communication (GSM), Code Division Multiple Access (CDMA, IS-95) deploy a cellular concept of network coverage for mobile stations (MS) by means of a plurality of fixed Radio Base Stations (RBS) provided with antennas.
In most cases, a base station provides its transmitting signal and retrieves its receiving signal from an antenna, mounted at an antenna mast. Antenna masts are usually placed at locations such that the one or more antennas, mounted in said mast, provide a predefined coverage of the radio cell, serviced by an RBS.
It will be understood that communication over the air interface within a cell, just like any radio propagation, might be subject to unexpected and undesired signals, interfering with the desired signals between RBS and mobile station (MS).
Undesired signals may be generated by interfering signal sources as e. g. other (unsynchronized) radio systems operating in the same bandwidth, or not properly suppressed equipment within the service area of the cell, having a transmit level that causes interference with the desired signals.
One of the solutions to this interference problem is to change the tilt of the antenna. For interfering signal sources close to the antenna, the antenna is tilted upward to reduce the nearby sensitivity. Likewise the antenna is tilted downward when the interfering signal source is in the outer part of the coverage area of the base station. Tilting of an antenna of an RBS as state of the art method to suppress interference has a disadvantage in that a decrease of sensitivity for interfering signal sources also affects sensitivity of the RBSs for reception of the signals from MSs.
Most commonly deployed RBSs are provided with a single mast with 3 antenna elements where each antenna element covers a sector of approximately 120 degrees, having some overlap between the beams of the adjacent antenna elements.
Even when encountering the fact that there is some overlap, tilting of an antenna element will have impact on a part of an antenna element beam pattern of at least 90 degrees resulting in a deviation of the desired coverage area of said antenna beam pattern where radio communication between RBSs and MSs could be reduced.
Tilting of antenna elements affects reception of mobile stations which limits the tilting in practice. This undesired limiting effect is even more extensive when the interfering signal source is present at the overlapping area of the radiation pattern of two antenna elements of the same mast, requiring both antenna elements to be tilted.